All disperse systems are 'held together' by mechanical and/or electrostatic forces between particles. They are not held together by formal chemical bonds. There are sevaral possible mechanisms available and at work in different disperse systems.

By thermodynamically unstable we mean that the (Gibbs) free energy of disassociation is negative.
This is negative because the disperse system is characterised by a very large surface area of interaction.

Thermodynamically

ΔG = ∂A

Where ΔG is the change in free energy, is the surface tension at the interface
∂A is the change in surface area

A reduction in surface area leads to negative ∂A and hence negative ΔG.

However if we take any solid of non spherical shape this is true, but does not mean that the solid is about to spontaneously collapse into a ball.